In recent times, much research effort has been done on solar heat collecting systems. This is primarily so because of the existing fact of a shortage of fossil fuels. Typical solar heat collecting systems comprise a solar collecting panel having an exteriorally exposed sunlight transmitting panel and at least one interiorally disposed sunlight transmitting panel, a solar collecting plate spaced apart from and behind the sunlight transmitting panels and a heat absorption fluid passing behind the collector plate to provide heat exchange with the collector plate itself. In actual operation, the sunlight passes through the sunlight transmitting panels and hits the collector plate. The collector plate converts the solar energy into heat energy; and, the heat absorption fluid which usually passes over one of the surfaces of the collector plate, provides heat exchange with the thermally warmed collector plate. Thereafter, the now heated absorption fluid is typically conveyed away to a remote place within the building structure for storage until it is subsequently utilized to warm the interior of the building.
The collector plates, utilized to convert solar energy to heat energy, are well known and are usually comprised of any suitable metal or like material of high solar absorptivity and are typically painted a dark highly absorptive color such as flat black. Metal from which satisfactory collector plates can be made are copper, aluminum, steel and galvanized iron. Aluminum is perhaps the most commonly used collector.
Heat absorption fluids which pass over the collector plate to provide heat exchange therewith are also known. Typical examples of heat absorption fluids utilized in solar heating systems are water, air, ethylene glycol, propylene glycol and other heat exchange fluids.
The heat produced during the light of the day must be stored so that it can be utilized to provide heating during the night and at times when the sun is not shining. This invention is concerned with an efficient heat storage system.
Many heat storage materials utilized to store heat energy for solar heating systems are phase change materials. That is, the heat storage material undergoes phase changes including changes from solid to liquid form and changes from one crystalline form to another during heat exchange as the heat storage material gains and loses heat. A typical example of a commonly utilized heat storage material which is a phase change material is Glauber's salt.
Normally one would not be concerned with whether or not a heat storage material undergoes phase changes during heat exchange. However, it has been found that many of the more efficient heat storage materials such as Glauber's salt, which have extremely desirable heat storage characteristics, also possess some unique problems. During the fluctuations in temperature involved in the heat exchange between the storage material and the heat absorption fluid, materials such as Glauber's salt undergo stratification. As a result, they form layers of crusted material. This stratification may reduce the heat storage capacity of the material such as Glauber's salt, by more than 50% in a relatively few cycles. Thus, many heat storage materials having extremely desirable heat storage characteristics cannot be utilized successfully because of their inherent stratification problems.
Sodium sulfate decahydrate, commonly known as Glauber's salt, is one example among a class of incongruently melting salts generally referred to as eutectic salts. Others include those listed in the table below.
TABLE 1 ______________________________________ EUTECTIC SALTS Melting Temp Density Heat of Fusion Storage Medium .degree.F. lb/ft.sup.3 Btu/lb Btu/ft.sup.3 ______________________________________ Na.sub.3 PO.sub.4 . 12H.sub.2 O 150 89 82 7,300 NaOH . H.sub.2 O 148 105 117 12,200 NaC.sub.2 H.sub.3 O.sub.2 . 3H.sub.2 O 136 81 114 9,200 Na.sub.2 S.sub.2 O.sub.3 . 5H.sub.2 O 119 103 90 9,300 Ca(NO.sub.3).sub.2 . 4H.sub.2 O 117 116 66 7,650 P116 Paraffin Wax 116 49 90 4,400 FeCl.sub.3 . 6H.sub.2 O 97 101 96 9,700 Na.sub.2 CO.sub.3 . 12H.sub.2 O 97 95 114 10,800 Na.sub.2 CO.sub.3 . 10H.sub.2 O 93 90 108 9,750 Na.sub.2 SO.sub.4 . 10H.sub.2 O 89 91 108 9,850 ______________________________________
As previously mentioned during the phase change from solid to liquid, these eutectic salts, and particularly Glauber's salt, are highly useful since the phase change involves a tremendous amount of heat which is either absorbed or rejected during phase change. This heat storage capacity is called "latent heat".
Again, as heretofore mentioned, incongruently melting eutectic salts exhibit a detrimental characteristic called stratification which results in a loss of latent heat storage capacity. For example, with sodium sulfate decahydrate, Glauber's salt, at 90.3.degree. F. the solid crystals melt and change to a saturated solution of 85% sodium sulfate in water and 15% anhydrous sodium sulfate. The anhydrous salt is more dense than the solution; consequently the white granular anhydrous salt settles to the bottom of the container. Increasing the temperature of the solution does not increase solution solubility. Upon continued cycling, three distinct layers can be identified within the container. The bottom layer will be anhydrous sodium sulfate unable to mix with the water, the next layer will be the decahydrate crystals and the top layer will be free water unable to mix with the bottom anhydrous layer.
In the past, there have been several approaches to solving this problem, all meeting with only limited success. For example, wood pulp and sawdust have been experimented with as well as organic thickener, such as silica gel starch and similar gel formers. Such biodegradable organic thickeners are not satisfactory in that they tend to decompose after long periods of time. As a result, stratification is no longer prevented. This invention involves a means of eliminating stratification problems of many desirable heat storage eutectic salts.
Accordingly, an object of this invention is to provide means for utilizing extremely desirable heat storage materials such as Glauber's salt which will avoid the stratification of that material for extended periods of time.
Yet another object of this invention is to provide heat storage articles, which when placed in a storage room allow for adequate air circulation around the individual containers to allow for efficient heat transfer.
Yet another object of this invention is to provide a heat storage unit for solar heating systems which not only provides a maximum heat exchange efficiency without causing stratification problems, but which provides a container which can be used for extended periods of time without detrimental effects caused by biodegradability of the filler materials used in combination with the eutectic salt to prevent stratification.
The method and means of accomplishing these objects, as well as others, will become apparent from the following detailed description of the invention.